High frequency switching off-line AC-to-DC converters are known to possess a relatively poor input power factor and display significant input current harmonic distortion. Large input current surges result from the switching action of the AC rectifying bridge along with the low impedance of the input filter capacitor. Typical power factor for off-line AC-to-DC converters is 0.65. To improve power factor and reduce the input current harmonic distortion, active power factor correction (APFC referred to simply as PFC) techniques have been developed. The PFC technique may be implemented by use of an AC-to-DC converter whose switches are controlled such that the input current follows the same sinusoidal shape as the input voltage. By controlling the input current, the input power can be controlled.
Assuming that the input voltage is sinusoidal with frequency f.sub.l, the input power P.sub.in will be proportional to a sin.sup.2 (2 f.sub.l t) function. As a result, to provide power to a load during the low portion of the input voltage, energy must be stored within the PFC converter. FIG. 1 shows this configuration. Typically, energy is stored in a large capacitor (105) at the output of the PFC converter. When the input line voltage is low, the load (104) draws its power from this energy storage capacitor (105), causing its voltage to droop. When the input line voltage is at a peak, excess current flows through the PFC converter (103) and into the energy storage capacitor (105) causing its voltage to rise. The frequency of the voltage ripple present across the energy storage capacitor (105) (as well as the load (104) )is twice input voltage frequency. This voltage ripple may be decreased by increasing the size of the capacitor, but this will degrade the dynamic response of the converter as well as increase cost and size.
In some applications, low frequency voltage ripple and slow dynamic response is tolerable, in which case the configuration in FIG. 1 is acceptable. However, in many situations, a tightly regulated output voltage is required. Therefor, fast dynamic response is required for responding to line voltage changes and/or load changes. With only a PFC converter, the input power is controlled, however the output power is unregulated. Naturally, by power conservation, the average input power must equal the average output power, however the instantaneous power flowing to the load is determined only by the instantaneous voltage across the energy storage capacitor. If the load is resistive with resistance R.sub.o, then the instantaneous output power will simply equal V.sub.o.sup.2 /R.sub.o. To tightly regulate the output voltage, the instantaneous output power must be controlled.
This may be accomplished by the addition of a DC-to-DC converter following the energy storage capacitor. This configuration is shown in FIG. 2. In this configuration, the PFC converter (203) is again controlled so that the input current follows the shape of the input line voltage, and the added DC-to-DC converter (205) is controlled in such a way that the output voltage is tightly regulated. If the load suddenly demands more power, the DC-to-DC converter (205) responds quickly, drawing more energy from the energy storage capacitor (204). If more energy is drawn from the capacitor (204) than is supplied by the PFC converter (203), its voltage will begin to droop. This decrease in voltage is sensed by the PFC converter (203) which in turn causes the input current to increase in magnitude all the while following the shape of the sinusoidal input voltage.
A favorable aspect of this configuration is that the two control loops for regulating the input current and regulating the output voltage are independent. When the instantaneous input power is greater than the instantaneous output power, energy will flow into the energy storage capacitor (204). If the converse is true, then energy will flow from the energy storage capacitor (204) supplying sufficient power for the second DC-to-DC converter (205). Although very widely used, this configuration has a drawback: The average power flowing to the output must be processed by two converter stages--the PFC converter (203), and the DC-to-DC converter (205). This leads to lower efficiencies as well as a larger power supply.
Recently, efforts have been made to integrate these two power stages into a single converter. Some methods attempt to use the same set of power switches for controlling the input as well as the output power, thereby meeting both objectives. One such method is shown in FIG. 3 in which two DC-to-DC converters appear. When implemented, these two converters (303, 305) may be constructed to share their power switches thereby reducing the number of components.
For purpose of explanation, these two converters have been distinguished one from the other in FIG. 3. The first DC-to-DC converter (303)is placed in series with the input rectifier bridge (302) so that its output current corresponds to the input current of the power supply. By controlling the output current of this converter (303), the input rectified AC current may be controlled thereby achieving power factor correction. This input current flows into the energy storage capacitor (304) which supplies both DC-to-DC converters (303, 305). The second DC-to-DC converter (305) is controlled to supply a constant voltage to the load (306). The advantage of this technique is that the first DC-to-DC converter (303) may be rated for a power rating as small as 13.7% of the output power. This occurs if the voltage across the energy storage capacitor (304) is equal to the peak of the input AC line voltage.
Typically however, the line voltage will tend to vary significantly leading to one of two things: Either the voltage across the energy storage capacitor (304) will vary with the peak of the AC line voltage forcing the second DC-to-DC converter (305) to be designed for wide input voltage range, or the voltage across the energy storage capacitor (304) can be fixed at the peak of the highest input AC line voltage causing excessive power (more than twice the average output power) to circulate through the first DC-to-DC converter (303) when the input is at low line.
Some converters possess an input impedance which closely resembles that of a resistor. In these converters, the peak input current inherently tends to be proportional to the input line voltage. When designed properly, the average input current will also closely follow the shape of the input voltage. As a result, no special control of the power switch(es) is needed to achieve a high power factor. If energy may be somehow stored in the converter, then a tightly regulated output voltage may be achieved by any conventional technique such as voltage mode, current mode, or charge mode control. This configuration appears identical to that of FIG. 1. The difference is the actual PFC converter (103) topology used.
Typically, the converters which manifest a relatively voltage independent input impedance are either resonant converters, or converters operated in discontinuous conduction mode. That is to say that the input current flowing through an input inductor falls to zero by the end of each switching cycle. Unfortunately, in resonant converters, the power factor is limited due to the sensitivity of the input impedance to input voltage variations. One the other hand, converters operated in discontinuous conduction mode exhibit higher power factor, but they are limited in power level due to high rms currents.
Another AC-to-DC technique has recently been proposed and is shown in FIG. 4. This technique minimizes the amount of power processed by each converter stage. Two power converters exist: a first DC-to-DC converter (403) is controlled primarily as a PFC preregulator, and a second converter (406) is controlled to regulate the output voltage. The PFC converter (403) is operated so that the input current follows the shape of the input voltage, however in addition, the switches of the PFC may control which of two paths the input power may take. Either, the input power may flow directly to the output load (407), or it may flow to an energy storage capacitor (405) present at the input of the second DC-to-DC converter (406). When the input power is greater than the desired output power, the input power is split into two paths during each switching cycle. If switch S.sub.p (404) is in position 1, then the input power flows directly to the output load (407). If however, switch S.sub.p (404) is in position 2, the input power flows to the energy storage capacitor C.sub.s (405).
Assuming that a Boost type topology is used for the PFC converter (403), we may conclude that power flows from the PFC (403) for a period of D'T.sub.s each switching cycle, where D' is the off-time of the Boost switch(es) divided by the switching period T.sub.s. If for example, the average input power over a switching period T.sub.s is 150% greater than the output power over that same period, then switch S.sub.p would direct power to the load for 66.7% of the period D'T.sub.s, and for the remaining 33.3% of the period D'T.sub.s, power would be directed to the energy storage capacitor (405). In this way the average output power over each switching period T.sub.s may be constant. During this time the second DC-to-DC converter (406) would be inactive since sufficient power for the load (407) could be provided by the AC input.
If however, the average input power over a switching period T.sub.s is less than the average output power over the same period, then switch S.sub.p (404) remains in position 1 so that all of the input power is directed to the output. In addition, during this period the second DC-to-DC converter (406) is operated such that it supplies additional power to the output load (407) for maintaining the average output power constant over each switching cycle. Naturally, the energy stored in the energy storage capacitor (405) during the peak portion of the input power supplies the second DC-to-DC converter (406) with energy when the input power is less than the output power.
The advantage of this technique is that part of the first DC-to-DC converter (403) handles only 68% of the average input power, while the second DC-to-DC converter (406) handles only 32% of the average input power. As a result, overall power conversion efficiency as well as power density may be increased. The draw-backs to this technique concern the complexity of control involved in achieving high input power factor as well as a tightly regulated output. In addition, two safety-approved isolation transformers are required, one for each converter (403, 406).